Desorption process and apparatus

ABSTRACT

Described are preferred processes and apparatuses for thermally desorbing desired chemical products from resins to which they are adsorbed. The processes and apparatuses provide highly efficient use of applied heat throughout resin preheat, desorption and cooling phases.

BACKGROUND OF THE INVENTION

The present invention relates generally to solid-phaseadsorption/desorption techniques for recovering valuable chemicalproducts. More particularly, the invention relates to highly efficientand economic processes for thermally desorbing adsorbed products, andapparatuses useful in conducting the processes.

As further background, the recovery and purification of carboxylic acidsand other valuable chemical products from mediums has long been studiedin an effort to discover efficient, cost-effective routes for theirproduction. For example, carboxylic acids such as citric acid and lacticacid are manufactured by fermentation in large scale worldwide. Suchfermentations provide fermentation broths from which the desired acidmust be recovered and purified. Where high volume manufacture isinvolved, the importance of keeping recovery costs to a minimum cannotbe overemphasized.

Recent recovery work has focused on the use of solid polymeric adsorbentmaterials to recover carboxylic acids from fermentation mediums. In thisapproach, the fermentation broth is passed over the adsorbent whichadsorbs the carboxylic acid, and the carboxylic acid is desorbed in somefashion to provide product. Generally, a wide variety of adsorbents andadsorption/desorption schemes have been proposed.

For example, Kawabata et al., U.S. Pat. No. 4,323,702, describe aprocess for recovering carboxylic acids with a material of which themain component is a polymeric adsorbent having a pyridine skeletalstructure and a cross-linked structure. The carboxylic acid is adsorbedon the adsorbent, and then desorbed using a polar organic material suchas an aliphatic alcohol, ketone or ester. However, these polar organicscan be difficult to separate from the eluted medium, and/or can causesignificant side reaction(s) during operations such as distillationnecessary for the separation.

Kulprathipanja et al., in U.S. Pat. Nos. 4,720,579, 4,851,573,4,851,574, teach solid polymeric adsorbents including a neutral,noniogenic, macroreticular, water-insoluble cross-linkedstyrene-poly(vinyl)benzene, a cross-linked acrylic or styrene resinmatrix having attached tertiary amine functional groups or pyridinefunctional groups, or a cross-linked acrylic or styrene resin matrixhaving attached aliphatic quaternary amine functional groups.

In their work, Kulprathipanja et al. describe "pulse tests" in whichthey identify acetone/water, sulfuric acid, and water as desorbents.Needless to say, an acetone/water desorbent leads to organic materialsin the desorbed fraction and attendant disadvantages as discussed above.When sulfuric acid is used as desorbent, it of course is present in theeluted fraction and complicates product recovery. Moreover, althoughthey name water as a potential desorbent, Kulprathipanja et al. indicateits unfeasibility in their processes, directing in their '573 patentthat water "is not strong enough to recover the absorbed citric acidquickly enough to make the process commercially attractive."

South African Patent Application No. 855155, filed Jul. 9, 1985,describes processes in which product acids were recovered from theiraqueous solutions. In the adsorption step, the acid-containing solutionwas passed through a column containing an adsorber resin consisting of avinylimidazole/methylene-bis-acrylamide polymer, avinylpyridine/trimethylolpropane tri-methacrylate/vinyltrimethylsilanepolymer, avinylimidazole/N-vinyl-N-methylacetamide/methylene-bis-acrylamidepolymer, Amberlite IRA 35 (Rohm & Haas--acrylate/divinylbenzene basedpolymer containing dimethylamino groups), or Amberlite IRA 93 SP (Rohm &Haas) or Dowex MWA-1 or WGR-2 (Dow Chemical) (these latter three beingstyrene/divinylbenzene based polymers containing dimethylamino groups).To desorb the acid, water, usually at a temperature of 90° C., wasallowed to pass through the column. However, the single-pass elutionprocess described involves an inefficient use of heat energy and doesnot substantially maximize the potential use of the resins to achievehighly concentrated desorbed solutions. Additionally, resins employed inthis South African application are relatively thermally instable andthus substantially degrade during desorption procedures employing hotwater.

International Applications PCT/US92/02107 filed Mar. 12, 1992 (publishedOct. 1, 1992, WO 92/16534) and PCT/US92/01986 filed Mar. 12, 1992(published Oct. 1, 1992, WO 92/16490) both by Reilly Industries, Inc.,disclose desorbing lactic and citric acid, respectively, fromdivinylbenzene crosslinked vinylpyridine or other resins using steam orhot water. The resins employed have advantageous adsorption/desorptioncapacities and are highly thermally stable under the described hot waterdesorption procedures. Nonetheless, improved processes would providegreater efficiency in the use of heat applied to the desorption andwould readily provide desorbed solutions of even higher productconcentration.

In light of this and other background in the area, there remains a needfor improved, effective processes for recovering carboxylic acids andother valuable products from their dilute solutions. The presentinvention addresses these needs.

SUMMARY OF THE INVENTION

Accordingly, preferred embodiments of the invention provide desorptionprocesses in which thermal energy is efficiently utilized in the thermaldesorption of resin-adsorbed products, and/or in which resin rinse orwash operations are conducted with product mediums to reduce productlosses while nonetheless providing an effective rinse. Thus, inaccordance with one aspect of the invention, a thermal desorptionprocess includes establishing a process wherein an amount of adsorbentis substantially fully loaded with an adsorbed product, the loadedadsorbent is rinsed with a rinse agent, and the loaded adsorbent is thentreated with a heated desorbing agent to form a product-containingmedium. In the inventive processes the rinse agent in these stepscontains an amount of the product to promote retention of (i.e. decreaseremoval of) the adsorbed product on the adsorbent, and theproduct-containing medium is passed through a heated heat exchanger andinto the same or another amount of product-loaded adsorbent, to furtherenrich the product-containing medium in the product.

Another preferred aspect of the invention provides a thermal desorptionprocess which includes establishing a process wherein an amount ofadsorbent is substantially fully loaded with an adsorbed product, theloaded adsorbent is rinsed with a rinse agent, and the loaded adsorbentis then treated with a heated desorbing agent to form aproduct-containing medium. In accordance with the invention theproduct-containing medium is passed through a heated heat exchanger andinto the same or another amount of product-loaded adsorbent, to enrichthe product-containing medium in the product. For instance, focusing onthe desorption step, processes in accordance with this embodiment caninclude a first desorption step conducted by passing a heated liquiddesorbent through a contacting zone containing a solid adsorbent resin,so as to desorb adsorbate or product from the resin. After the firstdesorption step, the product-containing desorbent is passed through aheat exchange zone in which additional heat is transferred to the liquiddesorbent. The heated desorbent is then subjected to a second desorptionstep in which it is passed through a contacting zone (the same oranother contacting zone) containing solid adsorbent resin to desorbproduct from the resin and further enrich the product-containingdesorbent in product.

Particularly preferred inventive processes employ a plurality ofresin-filled contacting zones (e.g. resin columns) loaded with product.The heated desorbent medium is passed through a first loaded column todesorb product and then through a heat exchange zone where additionalheat is transferred to the medium. The heated desorbent medium is thenpassed through a second loaded column to desorb product and become moreenriched in product. Prior to reaching the desorbent steps, theresin-filled contacting zones are preferably preheated by contact withheated, product-containing desorbent medium which is the product fromone or more previous desorption steps. This resin-preheating step alsoserves to cool the product-rich desorbent medium, thus making efficientuse of thermal energy in the system. Moreover, after the desorptionstep(s), the resin-filled contacting zone (now substantially stripped ofproduct) is preferably subjected to a cooling step so as to obtainoptimum temperatures for the adsorption zone in the next cycle. Thepreferred cooling step includes passing a liquid medium at a temperaturelower than the resin through the contacting zone, and then passing themedium through a heat exchange zone to remove heat from the medium. Onepreferred cooling step also includes using cold product-depleted feedsolution (waste) as the heat-transfer medium so as to make efficient useof the thermal energy in the system. Processes of the invention can becarried out in apparatuses appropriately valved to sequentially subjectthe contacting zones to preheat, desorption, and cooling steps.

Accordingly, a further preferred embodiment of the invention provides athermal desorption process which comprises the following steps:

(a) providing a plurality of chambers having inlet ports and outletports and containing a solid adsorbent resin loaded with product;

(b) advancing the chambers sequentially past a plurality of supply portsto cooperate with the inlet ports and discharge ports to cooperate withthe outlet ports;

(c) introducing a heated desorbent liquid into a first of the chambersthrough a first of the supply ports, the desorbent liquid passing overthe adsorbent resin in the first chamber and exiting through a first ofthe discharge ports as a first product-containing medium;

(d) passing the first product-containing medium after step (c) through aheat exchange zone wherein it is heated;

(e) conducting the heated medium after step (d) through a second of thesupply ports and into a second of the chambers, the product-containingmedium passing over the loaded adsorbent resin in the second chamber andexiting through a second of the discharge ports as a secondproduct-containing medium enriched in product as compared to the firstproduct-containing medium.

In accordance with another aspect of the invention, a desorption processincludes establishing a process wherein a plurality of contacting zonescontaining adsorbent are sequentially processed, the processingincluding substantially fully loading the adsorbent with an adsorbedproduct, rinsing the adsorbent with a rinse agent, and then treating theadsorbent with a desorbing agent to form a product-containing medium. Inthis aspect of the invention, a portion of the product-containing mediumfrom a prior-processed contacting zone is included in the rinse agent inthe processing of a subsequent contacting zone to decrease removal ofthe adsorbed product from the adsorbent during the rinsing step.

Again, particularly preferred processes employ a plurality ofresin-filled contacting zones (e.g. resin columns) loaded with product,which are subjected to rinse and then desorption steps. Desorbentmedium, preferably heated, is passed through a first loaded column todesorb product and form a product-containing medium. A first portion ofthe product-containing medium can be isolated as product, and a secondportion of the product-containing medium is then passed through a secondproduct-loaded column in a rinse operation to remove undesired,non-adsorbed or lesser-desorbed materials from the resin. Prior toreaching the rinse operation, the second portion of theproduct-containing medium is preferably cooled or allowed to cool, forexample employing a cooled heat exchanger. As before, processes of thisaspect of the invention can also be carried out in apparatusesappropriately valved to sequentially subject the contacting zones topreheat, desorption, and cooling steps.

Accordingly, particularly preferred modes of carrying out this aspect ofthe invention comprise the following steps:

(a) providing a plurality of chambers having inlet ports and outletports and containing a solid adsorbent resin loaded with adsorbedproduct;

(b) advancing the chambers sequentially past a plurality of supply portsto cooperate with the inlet ports and discharge ports to cooperate withthe outlet ports;

(c) introducing a desorbent liquid into a first of the chambers througha first of the supply ports, the desorbent liquid passing over theadsorbent resin in the first chamber and exiting through a first of thedischarge ports as a product-containing medium;

(d) conducting a portion of the product-containing medium after step (c)through a second of the supply ports which precedes the first supplyport, and into a second of the chambers, the desorbent liquid passingover and rinsing the adsorbent resin in the second chamber and exitingthrough a second of the discharge ports.

Other preferred embodiments of the invention provide a desorptionapparatuses. One apparatus of the invention comprises a plurality ofresin columns containing adsorbent resin, and liquid-circulating meansfor passing desorbent liquid sequentially through the resin columns. Theliquid-circulating means includes at least a first heat exchangeradapted to heat liquid after exiting one of the resin columns and priorto entering another of the columns. The liquid-circulating means alsopreferably includes a second heat exchanger downstream of the first heatexchanger (i.e. which becomes associated with the resin columnssubsequent to the first heat exchanger) and which is adapted to coolliquid after exiting one of the columns and prior to entering another.Additional features and preferred apparatuses of the invention arediscussed below, and include further heat exchangers to preheat theresin columns as well as advantageous carousel devices incorporatingheat exchangers to achieve resin column preheating, desorption andcooling functions.

The inventive processes and apparatuses provide for the recovery ofproducts in concentrated liquid mediums while efficiently utilizingthermal energy to aid in the recovery, and also while operating inconfigurations which reduce product loss from wash operations and thusprovide more product-concentrated desorbed mediums. The invention alsoprovides processes and apparatuses which are easy to operate, and whichcan be employed to recover a wide variety of desired product products inconcentrated solutions. Additional preferred embodiments, features andadvantages of the invention will be apparent from the followingdescription.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a side elevational view partially in cross-section of anillustrative continuous contacting apparatus which can be used in thepresent invention.

FIG. 2 is a schematic diagram of a desorption section of a continuouscontacting apparatus which can be used in the present invention.

FIG. 3 is a schematic diagram illustrating a port configuration for acontinuous contacting apparatus which can be used in the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to certain of its embodiments andspecific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and modifications andapplications of the principles of the invention as described hereinbeing contemplated as would normally occur to one skilled in the art towhich the invention pertains.

As indicated above, a preferred process of the invention providesthermal desorption of an product from an adsorbent resin. As discussedpreviously, in general, adsorption/desorption processes for recoveringvaluable chemical products are known, and a wide variety of adsorbentresins and appropriate liquid desorbents have been identified. Thus,those ordinarily skilled in the art will be readily able to select anduse suitable adsorbent resins and desorbent mediums in accordance withthe present invention.

Generally speaking, the adsorbent employed will have the capacity adsorbthe desired product, and will be stable (i.e. will not undergosubstantial degradation) under the processing conditions utilized asfurther described below. Although the adsorbent can be any substancecapable of adsorbing the desired product (including activated carbon,zeolites, etc.), more desirable adsorbents for use in the invention willbe polymeric adsorbent resins which are crosslinked to provide thermaland mechanical stability and an advantageous physical form. Bead-formadsorbent resins are preferred, especially those having a particle sizeof about 20 to about 100 mesh, more especially about 40 to about 60mesh.

A wide variety of suitable polymeric adsorbents have been reported forthe recovery of desired adsorbate products such as carboxylic acids.These polymeric adsorbent are formed through the polymerization of oneor more monomers usually including a crosslinking monomer. Thepolymerization is carried out to provide resin beads in either gel ormacroreticular form. Further, these resin beads can be chemicallymodified, e.g. by adding ionic groups to the resin such as quaternarysalt or acid salt groups. The resulting resin can thus be anionic,cationic or nonionic and have a variety of physical and chemicalcharacteristics. This variety of polymers is known in the art for use asadsorbents. For example, suitable reported resins include nonionic andionic polymers, including neutral, noniogenic, macroreticular,water-insoluble cross-linked styrene-poly(vinyl)benzene, basic polymermaterials such as crosslinked pyridine-containing polymers, e.g.vinylpyridine polymers, cross-linked acrylic or styrene resin matriceshaving attached tertiary amine functional groups or pyridine functionalgroups, cross-linked acrylic or styrene resin matrices having attachedaliphatic quaternary amine functional groups, and the like (see e.g.Kawabata and Kulprathipanja et al. patents cited in the Background).These and numerous other polymers known in the art for use asadsorbents, for example base or acid ion exchange resins includingstyrenic, acrylic, epoxy amine, styrene polyamine, and phenolformaldehyde, or nonionic adsorption resins, will be suitable for use intemperature swing adsorption/desorption processes in accordance with theinvention; preferably, however, the polymer adsorbent will be acrosslinked base polymer, such as a crosslinked polymer containingN-aliphatic or N-heterocyclic tertiary amine functions, for examplepolymers containing dialkylamino or pyridine functionalities.

Particularly preferred pyridine-containing polymers arepolyvinylpyridine polymers such as poly 2- and poly 4-vinylpyridine gelor macroreticular resins exhibiting a bead form. These resins arepreferably at least about 2% cross-linked with a suitable cross-linkingagent, desirably a divinyl crosslinking agent (i.e. a crosslinking agenthaving two vinyl moieties) such as divinylbenzene. More preferred resinsare 2 to 25% crosslinked bead-form vinylpyridine polymers, e.g. poly 2-and poly 4-vinylpyridine polymers.

For example, more preferred resins include poly 2- and poly4-vinylpyridine resins available from Reilly Industries, Inc.,Indianapolis, Ind., in the REILLEX™ polymer series. These REILLEX™polymers are 2% or 25% crosslinked, and exhibit good thermal stabilityand adsorptive and desorptive capacities and other preferred features asdescribed herein. For example, preferred resins of this type haveexhibited desorptive capacities of at least about 200 mg citric acid pergram of polymer. Additional preferred resins are available from thissame source under the REILLEX™ HP polymer series. These REILLEX™ HPpolymers also exhibit advantageous capacities, and are highlyregenerable. For more information about these REILLEX™ polymers,reference can be made to the literature, including that available fromReilly Industries, Inc. in the form of REILLEX™ reports 1, 2 and 3.

AMBERLYST A-21 resin from Rohm and Haas, Philadelphia, Pa. can also beused in the invention. This A-21 resin is crosslinked by divinylbenzene(greater than 2%) and contains aliphatic tertiary amine functions(particularly, attached dialkylamino (dimethylamino) groups). Foradditional information about this and other similar resins, referencecan be made to the literature including that available from themanufacturer. See, e.g., AMBERLYST A-21 technical bulletin fluid processchemicals," Rohm and Haas, April 1977.

In accordance with the invention, the method by which the product isadsorbed onto the resin can be conventional. In some fashion or another,methods for loading the adsorbent with product involve contacting theresin with a liquid (usually but not necessarily aqueous) mediumcontaining product under appropriate conditions for adsorption. Thiscontacting may be performed in any suitable apparatus, for example incolumns containing the adsorbent resin through the medium is passed, asfurther discussed below. It is preferred to substantially load theadsorbent resin with product, that is, to continue loading the resinwith product until the resin's adsorptive capacity is substantiallyexhausted. For example, in preferred inventive processes, the adsorbentresin will be at least 50% loaded with product (i.e. 50% of the resin'stotal capacity to adsorb the product is depleted), more preferably atleast 80% loaded with product. In general, higher levels of loading willresult in a more effective uses of the resin; however, it will beunderstood that the most efficient and economic loading levels may varyfrom process to process and determining and achieving such levels willfall within the purview of the skilled worker given the teachingsherein.

Thermal desorption processes of the invention will generally findutility if the heat of reaction between the product and the adsorbent issignificant, e.g. such that the adsorbent has a substantially greatercapacity to adsorb the adsorbent at relatively low temperatures ascompared to higher temperatures. Of particular interest in the inventionis the recovery of organic acids, especially carboxylic acids such asaliphatic carboxylic acids, from mediums in which they have beenproduced by fermentation, i.e. by the fermentation of suitable carbonsources by microorganisms. The commercial success of such fermentationprocesses depends heavily upon the ability to effectively recoverproduct acids from their formed, relatively dilute solutions.

For example, substantial worldwide production of organic carboxylicacids such as citric and lactic acids is performed by fermentation. Inthe case of citric acid, the broth may be from a fermentation of acarbon source such as corn sugar or molasses with a suitablecitric-acid-producing bacteria or other microorganism such asAspergillus niger. Lactic acid is produced using bacteria or othermicroorganisms capable of forming lactic acid upon metabolizing a carbonsource. Typically, bacteria of the Family Lactobacillaceae are employed,although other microorganisms such as fungi may be used. For example,fungi of the family Rhizopus, such as Rhizopus oryzae NRRL 395 (UnitedStates Department of Agriculture, Peoria, Ill.), can be employed toproduce substantially pure L+ lactic acid as generally taught inInternational Application No. PCT/US92107738 filed Sep. 14, 1992 byReilly Industries, Inc. (published Apr. 1, 1993, WO 93/06226). It iswell within the purview of the skilled artisan to select and usesuitable fermentation organisms to produce fermentation brothscontaining organic acids such as carboxylic acids, which broths can betreated in accordance with the invention to recover the acids.

When a carboxylic acid-containing fermentation medium is involved, itwill usually contain water, the product acid, salts, amino acids,sugars, and other various components in minor amounts. Such fermentationmediums can be filtered to remove suspended solids prior to theadsorption step. Similarly, after loading the adsorbent with theproduct, it will often be desirable to rinse the loaded polymer withcold water or another suitable agent to wash away any salts or othermaterials desirably kept out of the desorbed product-enriched medium. Ofcourse, the rinse temperature, agent and other factors will be designedto maximize removal of undesired residues, and minimize removal of theproduct of interest, from the resin bed. These and similar details inthe general practice of the invention will readily occur to thosepracticed in the relevant field.

A feature of the invention, however, relates to the discovery of processconfigurations which can be used to increase the concentration of thedesired product in the product streams and minimize product loss towaste. Generally in these configurations, after the adsorbent resin hasbeen loaded with product, the loaded resin bed is rinsed using a rinsemedium containing the product. It has been discovered that such productstreams, for example resultant of a prior desorption operation torecover the product, can be used to effectively rinse undesired,non-adsorbed or relatively weakly-adsorbed residues such as sugars fromthe resin bed. At the same time, the presence of product in the rinseagent has been found to increase retention of adsorbed product on theresin during the rinsing operation, thus leading to increasedefficiencies in the use of the resin capacity for the product andincreased concentrations of the product in the final product stream.Thus, in preferred operations a portion of the product stream from theoverall adsorption-rinse-desorption process can be diverted to the rinsestep in order to provide an increase in concentration of the product inthe desorbed stream.

A variety of liquid desorbents or desorbing agents can be employed inthe present invention. These desorbents include, for example, organicsolvents, e.g. polar organic solvents such as alcohols, ketones andesters, as well as aqueous mediums such as water (i.e. substantiallypure water without added solutes), aqueous solutions of acids or bases,e.g. hydrochloric acid, sulfuric acid or sodium hydroxide solutions, orwater/organic co-solvent mediums such as water/alcohol mixtures. Morepreferred inventive processes employ water so as to provide desorbatemediums free from unnecessary solutes or co-solvents which maycomplicate recovery of the desired product.

Generally, thermal desorptions of the invention will occur at elevatedtemperatures sufficient to desorb the product from the adsorbent resin,and at which the resin and product are thermally stable. In preferredprocesses utilizing aqueous desorbents such as water, the desorptiontemperature will generally be above about 50° C., more typically aboveabout 70° C. and often above about 90° C. When recovering carboxylicacids such as citric or lactic acid, desorption temperatures above about90° C. will be preferred.

Favored processes of the invention are conducted using a continuouscontacting apparatus ("CCA"). For example, continuous contactingapparatuses which are useful in the invention include those such as theISEP Continuous Contactor available from Advanced SeparationsTechnology, Inc. (AST, Inc.), Lakeland, Fla., and are also generallydescribed in U.S. Pat. No. 4,764,276 issued Aug. 16, 1988, U.S. Pat. No.4,808,317 issued Feb. 28, 1989 and 4,522,726 issued Jun. 11, 1985. Abrief description of such a CCA device as described in these patents isset forth below. For further details as to the design and operation ofCCA's suitable for use in the invention, reference can be made toliterature available from AST, Inc. including "The ISEP™ Principle OfContinuous Adsorption", and as well to the above-cited U.S. patents.

The preferred CCA for use in the present invention will be aliquid-solid contact apparatus including a plurality of chambers whichare adapted to receive solid adsorbent material. The chambers haverespective inlet and outlet ports, and are mounted for rotation about acentral axis so as to advance the chambers past supply and dischargeports which cooperate with the inlet and outlet ports. In particular,liquid is supplied individually to inlet ports at the top of thesechambers through conduits connected with a valve assembly above thechambers, which valve assembly provides a plurality of supply portswhich cooperate with inlet ports of the chambers as they are advanced.Similarly, conduits connect the outlet port at the lower end of eachchamber with a valve assembly below the chambers which providesdischarge ports which cooperate with the outlet ports as the chambersare advanced. The valve assemblies include movable plates with slotsthat cover and uncover inlet ports as the plate rotates with thecarousel. By varying the size of the slots in the plate and the locationof the slots, the flow from the supply conduits into the chamber andflow from the chamber to the exhaust conduits can be controlled in apredetermined manner. The time during which liquid flows into and out ofthe chambers is a function of the speed of rotation of the chambersabout the central axis.

More specifically, a preferred contacting device for use in theinvention is shown generally in FIG. 1. The apparatus includes arectangular frame 11 which supports a vertical drive shaft 12. Acarousel 13 is mounted for rotation on the drive shaft. The carousel isfixed to the shaft and the shaft is driven by a motor 14 mounted on theframe 11. A plurality of cylindrical chambers 15 (e.g. 30 chambers) aremounted vertically on the carousel 13. The chambers are preferablyarranged in staggered relation around the circumference of the carousel.Each of the chambers is filled with resin or other suitable solidadsorbent material according to the particular process being performed.As shown at the left side of FIG. 1 in cross-section, the solidadsorbent material 16 is preferably filled to about one-half the heightof the chamber 15. An arrangement is provided on each chamber 15 forinserting and removing the solid material through the top of thecontainer. Pipe fittings 17 and 18 are provided at inlet and outletopenings on the top and bottom, respectively, of each chamber 15. Anupper valve body 19 and a lower valve body 20 are mounted over the driveshaft 12. The valve bodies 19 and 20 provide supply and discharge ports,respectively (e.g. 20 each). Individual conduits 21 and 22 connect thevalve bodies 19 and 20 with the respective upper and lower pipe fittings17 and 18, so as to allow cooperation of the supply and discharge portsof valve bodies 19 and 20 with the inlet and outlet ports of thechambers 15. Supply conduits 23 are mounted in the top of the frame 11and extend upwardly from the valve body 19. Similarly, dischargeconduits 24 extend downwardly from the lower valve body 19 to the frame11. In this manner, as the carousel is rotated to advance the chambers15, the inlet and outlet ports of the chambers 15 cooperate with thesupply and discharge ports of the valve bodies 19 and 20 to provideadvantageous means for circulating liquids through the chambers 15.

In accordance with one aspect of the invention, the apparatus of FIG. 1will preferably be configured so as to include adsorption, rinse, anddesorption zones, and optionally a regeneration zone. The adsorptionzone can be conventionally operated so as to adsorb the material ofinterest, e.g. citric acid, onto an adsorptive resin contained withinchambers 15 from a feed solution. Generally, a feed solution containingthe citric acid will be passed countercurrent through the chambers 15and over the resin so as to achieve substantial loading of the resinwith citric acid as described previously A rinse zone can also beestablished and conventionally operated, in which water or another rinsematerial is passed countercurrent through the chambers and over theresin beds therein to remove salts or other undesired residues from thebeds. It will be understood that the number of ports of the CCAdedicated to the adsorption and rinse zones may vary, and will bedetermined so as to maximize overall process economics. In addition, itwill be understood that the advantageous use of product stream in therinse zone as discussed above can be achieved by diverting a portion ofthe product from the desorption zone to the rinse zone. Specificillustrations of such configurations are discussed below in connectionwith the FIG. 3.

A feature of the present invention is the establishment of a desorptionzone in which the resin beds within chambers 15 are heated and/or cooledby recirculation of liquid at appropriate temperatures and atappropriate times, so as to effectuate heat conservation and therecovery of highly concentrated desorbed product mediums.

FIG. 2 is a schematic of one desorption zone configuration thatdemonstrates the aspects described above. This particular configurationcan be used, for example, in the recovery of citric acid from a citricacid broth. After adsorption and optionally rinse steps, the chamberscontaining the resin beds would be entering the desorption zone at portP10. At this point, the resin beds are cold, e.g. exhibitingtemperatures in the range of about 0° C. to about 30° C. The adsorbentresin will be loaded with citric acid, i.e. citric acid is adsorbed tothe resin. The resin is then contacted in a countercurrent manner withhot citric acid solution leaving the system in ports P9 and P10 (thishot citric acid solution is produced as discussed below). The resin bedis heated as it moves through the P10 port and then through the P9 port,having a temperature of about 60° C. to about 90° C. when leaving the P9port. The citric acid solution flows first through the P9 port exitingwith a temperature of about 65° C. to about 95° C. and then through theP10 port exiting with a temperature of about 30° C. to about 70° C.

Upon entering port P8 from port P9, the resin is heated to some extentas discussed above, but has not yet reached the desired desorptiontemperature. In port P8, a hot citric acid solution at the desiredtemperature, typically about 70° C. to about 100° C., is recirculated toheat the bed to the temperature of the liquid. Preferably, at least twoand one half bed volumes of heated citric acid solution are recirculatedthrough the resin bed while at port P8 to accomplish this heating. Theheated citric acid solution is obtained using a heat exchanger situatedbetween ports P7 and P8 and effective to transfer heat into thesolution. For example, in-line heat exchange can be accomplished byoperably associating a heat exchanger with conduit running between portsP7 and P8. Preferably, however, the solution exiting port P7 is fed intoa tank or other suitable reservoir "T1" equipped with a heat exchangersuch as a steam-fed or electrically heated coil. The solution iscollected and has a residence time in the tank sufficient to accomplishthe desired heat transfer. The thus heated citric acid solution is thenfed to port P8 to there heat the resin beds as discussed above. Morespecifically, as illustrated, upon exit from port P8, the citric acidsolution enters a chamber in T1 separated by an internal wall "IW" fromthe heat exchange side of T1. The chambers are sized and the internalwall "IW" has a height such that a portion of the citric acid solutionexiting port P8 spills back into the heat exchange side of T1 and aportion is fed on to port P9. Thus, at least a portion of the citricacid solution exiting port P8 is recirculated through the heat exchangezone in T1 and back into port P8 to further heat the resin bed therewithassociated.

After the bed is heated in port P8, it proceeds through ports P7 and P6where it is contacted with hot water at about the same temperature ofthe bed to extract or desorb the citric acid from the bed. Asillustrated, a further heat exchanger is positioned to transfer heat towater immediately prior to its reaching port P6. Similar to the heatexchanger discussed above, this heat exchanger can be either an in-lineexchanger or can be associated with a tank "T2" in which the exit feedfrom port P5 is collected.

As the bed continues through ports P5, P4, and P3, the bed is cooled bythe countercurrent passing of water and the water is heated up. At thispoint, the temperature of the resin moving from port P3 will range from5° C. to about 35° C. and the water exiting from port P5 will range fromabout 45° C. to about 80° C. Further washing of the resin is alsoaccomplished by this water. When the bed reaches port P2, it is stillnot at the desired cold temperature and therefore a recirculation of acold liquid (water in this case) is used to cool the bed to the desiredtemperature. Again, at least two and one half bed volumes of liquid arepreferably recirculated to accomplish the cooling of the bed. Asillustrated, a still further heat exchanger is positioned to remove heatfrom water prior to its reaching port P3. As with the heat exchangersdiscussed above, this heat exchanger can be either an in-line exchangeror can be associated with a tank "T3" having separate chambers intowhich the exit feeds from ports P1 and P2 are collected, respectfully.Similar to T1, T3 has a heat exchange chamber separated from anotherchamber by an internal wall "IW". The size of the chambers and theheight of the internal wall are such that a portion of the solutionexiting port P2 spills over into the heat exchange side of T3 wherein itis further cooled and recirculated back through port P2 to further coolthe resin bed therewith associated. The remainder of the solutionexiting port P2 is conducted on to port P3. In port P1, cold water isintroduced for the final wash of the citric acid from the resin.

It will be understood that the number of ports allocated to each of thefunctions in the desorption zone can be varied depending upon flow ratesrequired and relative heat capacities and heat transfer coefficients ofthe materials. For example, if the viscosity of any of the liquids istoo high to permit recirculating the two and one half bed volumes in oneport, it is possible to assign two ports to the recirculation. Forexample, in the case of recirculation heating it would be possible touse both ports P8 and P9 for this activity and leave only port P10 forheat transfer. Similarly, if ports P3, P4 and P5 are more than areneeded for the heating of the liquid while simultaneously cooling thebed, these ports could be reduced to e.g. P4 and P5.

Of course, after exiting the desorption zone, the resin beds canoptionally be subjected to a regeneration zone to prepare them for thenext adsorption function. The regeneration zone can also beconventionally operated to remove residuals from the resin. For example,where a resin containing tertiary amine functions is employed, the resinmay be treated with a basic material such as NaOH and subsequentlyrinsed with water.

It will be understood that it is not necessary to accomplish therecirculation cooling of the resin in the desorption or strippingsection of the CCA. Rather, it is possible to obtain the requiredcooling of the resin by recirculating the liquid at the start of theadsorption section in a similar manner as the heating is accomplishednear the start of the desorption section in port P8.

It will also be understood that the above-described heat and coolingconcepts can be employed with stripping operations other than purelytemperature adsorption/desorption processes. Ion exchange or solventregeneration also can be combined with the temperature process. Thus,for example, after the temperature desorption as shown in FIG. 2, theresin beds could proceed from port P1 into a chemical stripping section(e.g. base, acid or solvent) to remove other adsorbed species beforeagain entering the regeneration or adsorption section of the CCA.

Preferred processes of the invention conducted in the CCA will becarried out in a fashion which minimizes product loss and contaminantsin the product, while maximizing the concentration of product in theproduct stream. In many ways these are competing interests, as ingeneral terms many measures taken to increase the concentration ofproduct in the product stream and decrease product contaminants willresult in decreases in product recovery, and vice versa. For example,increasing the product concentration in the feed stream will generallylead to increased concentrations of product in the product stream;however, feed streams which are too rich in product will exceed theeffective capacity of the resin and result in significant product loss.Similarly, decreasing the rate at which rinse streams are fed throughthe resin beds will provide product streams with higher productconcentrations, but on the other hand can lead to increased contaminantsin such product streams due to ineffective rinsing.

Desorption operations as discussed above can provide surprising andunexpectedly high levels of products such as citric acid in therecovered desorbed medium. For example, citric acid solutions exceedingabout 12% by weight citric acid, and even up to about 14% or more citricacid, can be obtained using water as the desorbent, all while conservingand effectively utilizing heat energy applied to the desorptionfunction. These highly enriched product streams can typically beobtained using concentrated feeds, for example about a 30% citric acidfeed. However, fermentation broths often contain the product, forexample citric or lactic acid, in a lower concentration, say, about 10%to about 15%. Thus, pre-concentration of this broth would be required toobtain a 30% product feed. Such pre-concentration measures arerelatively costly, and are thus preferably avoided. In addition, asdiscussed above, highly pre-concentrated feeds can exceed the effectivecapacity of the resin and lead to product loss to waste.

As mentioned above, another feature of the invention relates to anadsorption-rinse-desorption process in which the product stream or atleast a portion thereof is utilized in the rinse operation. Thesepreferred processes provide an increase in the concentration of theproduct in the product stream, thus addressing the need for enrichedproduct streams without the requirement of feed pre-concentration. FIG.3 illustrates a configuration utilizing this aspect of the invention ina CCA such as that described above. In particular, runs utilizing theconfiguration of FIG. 3 were carried out in an ISEP L100 pilot-scale CCAavailable from AST, Inc. The operation of the device is essentially asdescribed above for CCA'S, with the device having a stationary manifoldwith 20 ports and a carousel of 30 resin columns cooperating with theports. The valving operation created isolates the columns from differentfluid streams while maintaining continuous flow through the ports. TheL100 pilot unit had glass columns (1 inch inner diameter, 350 mlvolume), with polypropylene caps and 70 mesh screening to contain theresin. The upper and lower valve assemblies connected to the columnswere constructed of polypropylene and 316 stainless steel. Allconnections throughout the apparatus were made with standard 1/4 inchinner diameter polypropylene, polyethylene or Teflon tubing. The heatexchangers illustrated in FIG. 3 were either tube-and-shell orplate-and-frame heat exchangers. Low pressure steam was used for heatingand chilled water (about 15° C.) or tap water (ambient temperature) wasused for cooling. Peristaltic pumps with norprene tubing of varioussizes (14-18) were used to transfer all solutions. The tubing wasinsulated in order to limit heat transfer with the surroundingenvironment.

Temperatures were measured in-line at the connection panel using 1/4inch stainless steel J-type thermocouples and recorded on a Yokogawa HR1300 chart recorder. Pressures were measured in-line at the connectionpanel using stainless steel gauges. Flow rates were measured manually byweight over time (generally 30 minutes) at the various input or outputports.

Anhydrous USP/FCC grade citric acid, maltose monohydrate and anhydrousD-(+)-glucose, mixed anomers, and deionized water were used to make upthe citric acid feed solutions. These solutions were preparedimmediately prior to use to avoid potential bacterial growth.

REILLEX HP™ polymer was obtained from Reilly Industries, Inc. in waterwet form. The resin was sieved to obtain beads in a size range of 30-60mesh. The resin was then soaked in 15% citric acid solution overnightand transferred to the L100 glass columns. Each column was backwashedwith several bed volumes of water to remove fines, then connected to theL100. A total of 10.5 L of citric acid swollen resin was charged to the30 columns, which equated to about 7.55 L of water swollen resin or 2.10kg dry resin.

FIG. 3 shows a schematic diagram of a preferred port configuration inthe L100 which was used in the illustrative citric acid runs. The columnrotation of the L100 was counter-current to solution flow, i.e. fromright to left in FIG. 3. The measured temperatures, flow rates andinsterstage tank positions are also shown in FIG. 3. Briefly describingthe configuration, the flow pattern through the columns is generallydownflow, with cold citric acid adsorption carried out in ports P5-P9,wash in ports P1-P4, and hot water desorption in ports P14-P17. PortsP10-P13 are used for cool-down of the resin after the desorption stage,while ports P18-P19 are used to preheat the loaded resins prior to thedesorption stage. Port 20 is used to push remaining wash solution fromthe resin column.

More particularly, citric acid feed solution is fed from tank 30 usingpump 31 through heat exchanger 32 (providing cooling) and into port P5.The stream exiting port P5 is fed to feed interstage tank 33. Solutionfrom feed interstage tank 33 is fed using pump 34 through heat exchanger35 (providing cooling) and into ports P6 and P7 in a parallel flowconfiguration. The streams exiting ports P6 and P7 is fed through heatexchanger 36 (providing cooling) and into ports P8 and P9 in parallelflow configuration. The streams exiting ports P8 and P9 are fed intoprecool/waste interstage tank 37. Solution from tank 37 is fed usingpump 38 through heat exchanger 39 (providing cooling) and into ports P10and P11, and the exit streams from ports P10 and P11 are directed backinto tank 37. This "pump-around" provides precooling of the resin bedsprior to their entry into the citric acid adsorption stage. Wasteoverflow also occurs at tank 37.

For the desorption stage, the desorption medium, e.g. water, is fed fromfeed tank 40 using pump 41 into ports P12 and P13 in parallel flowconfiguration. The exit streams from ports P12 and P13 flow into stripinterstage tank 42. Pump 43 feeds solutions from tank 42 through heatexchanger 44 (providing heating) and back into tank 42. This pump-aroundthrough the heat exchanger heats the solutions in tank 42 for thedesorption operations. It should be noted that strip interstage tank 42and the other tanks employed in the configuration are preferably open tothe atmosphere. This allows gasses evolved from the solutions,particularly when heated, to escape prior to entering the resin columns.This is advantageous for the reason that gasses flowing through theresin beds can cause channeling and interfere with the efficientoperation of the device.

Heated solution from strip interstage tank 42 is fed using pump 45 intoports P14 and P15 in parallel flow. The exit streams from P14 and P15are collected in midstrip interstage tank 46. Materials in tank 46 arealso subjected to pump-around using pump 47 and heat exchanger 48(providing heating). Materials from tank 46 are also fed to ports P16and P17 in parallel flow configuration, and the corresponding exitstreams are collected in preheat/product interstage tank 49. Materialsin tank 49 are also subjected to pump-around using pump 50 and heatexchanger 51 (providing heating). A portion of the materials in tank 49are recovered as product overflow. Another portion is fed using pump 52into ports P18 and P19, and the corresponding exit streams are fed backinto tank 49. The pump-around through P18 and P19 serves to preheat theresin beds prior to their entry into the desorption stages of theoperation. Another portion of the materials in tank 49 is fed using pump53 into port P20, which also preheats the resin beds at P20 to someextent.

In an important aspect of the present invention, the exit stream fromport P20 is fed through heat exchanger 54 (providing cooling) and intothe wash stage of the operation. Specifically, the port P20 exit streamis fed into ports P1 and P2 in parallel flow configuration. In theillustrated and preferred configuration, the product stream from portP20 serves exclusively as the wash agent in the wash operation. It willbe understood, however, that this product stream could be used inconjunction with other wash agents fed to the wash stage, for examplewater. The collected exit streams from ports P1 and P2 are fed inparallel flow configuration into ports P3 and P4. Thus, in ports P1-P4,resin beds loaded with product, e.g. citric acid, are rinsed to removenon- or lesser-adsorbed materials such as sugars. The exit streams fromports P3 and P4 are collected in feed interstage tank 33, where they mixwith the citric feed exiting port P5 and are processed along therewithas discussed beginning above.

In illustrative runs utilizing the configuration of FIG. 3, the L100unit was run continuously for 3 hours before samples were collected, tofacilitate the system reaching equilibrium or near equilibriumconditions prior to sampling. Output samples were collected over 30minutes to ensure proper representation, and when more than one samplewas taken, the time of collection was staggered with respect to therotation rate so streams were not repeatedly coming from the samecolumns. In one such run, extending over 28 hours, a 14.7% citric acidfeed containing 0.43% glucose and 2.03% maltose was fed to the system.The flow rates and temperatures of the flowing solutions at various keypoints in the system for this run are set out in FIG. 3. The productstream contained 9.73% citric acid on average (96% recovery) with 94%glucose and 97% maltose removal. In another similar run, except omittingthe heat exchanger 32 and thus the cooling of the feed to column 5, theproduct stream contained 9.76% citric acid (98% recovery), and glucoseand maltose removals were 96% and 98%, respectively. The processes thusprovided highly enriched citric acid product mediums, with low sugarlevels. These results compare very favorably to similar configurationsof the L100 unit except using water in the rinse operation instead ofproduct stream. In these water-wash runs, while the removal of sugarswas effectively achieved, the product streams contained about 5.5% to 7%citric acid and the recovery of citric acid was generally below 90%.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

All publications cited herein are indicative of the level of skill inthe art and are hereby incorporated by reference as if each had beenindividually incorporated by reference and fully set forth.

What is claimed is:
 1. A thermal desorption process,comprising:establishing a process wherein an amount of adsorbent issubstantially loaded with an adsorbed product, the loaded adsorbent isrinsed with a rinse agent, and the loaded adsorbent is then treated witha heated desorbing agent to form a product-containing medium; whereinthe rinse agent contains an amount of the product to promote retentionof the adsorbed product on the adsorbent; and wherein theproduct-containing medium is passed through a heated heat exchanger andthrough the same or another amount of product-loaded adsorbent, toenrich the product-containing medium in the product.
 2. The process ofclaim 1 wherein the product is a carboxylic acid.
 3. The process ofclaim 2 wherein the desorbing agent is an aqueous medium.
 4. The processof claim 3 wherein the product is citric acid or lactic acid and thedesorbing agent is water.
 5. The process of claim 4 wherein the productis citric acid.
 6. The process of claim 2 wherein the adsorbent is across-linked polymer resin containing tertiary amine groups.
 7. Theprocess of claim 4 wherein the adsorbent is a crosslinked polymer resincontaining tertiary amine groups.
 8. The process of claim 7 wherein theadsorbent is a pyridine-containing polymer resin.
 9. The process ofclaim 7 wherein the polymer resin is crosslinked with divinylbenzene.10. The process of claim 8 wherein the polymer resin is crosslinked withdivinylbenzene.
 11. The process of claim 10 wherein the polymer resin isa divinylbenzene crosslinked poly-2- or poly-4-vinylpyridine.
 12. Theprocess of claim 11 wherein the resin is in a macroreticular bead form.13. The process of claim 12 wherein the resin is about 2% to 25% byweight crosslinked with divinylbenzene.
 14. The process of claim 13wherein the product is citric acid.
 15. The process of claim 1, whichcomprises:establishing a process wherein a plurality of contacting zonescontaining adsorbent are sequentially processed, the processingincluding substantially loading the adsorbent in the contacting zonewith the adsorbed product, rinsing the adsorbent in the contacting zonewith a rinse agent, and then treating the adsorbent in the contactingzone with a desorbing agent to form a product-containing medium; whereina portion of the product-containing medium from a prior-processedcontacting zone is included in the rinse agent in the processing of asubsequent contacting zone, to decrease removal of the adsorbed productfrom the adsorbent during the rinsing step.
 16. A thermal desorptionprocess, comprising:(a) desorbing a product from a solid adsorbentresin, said desorbing including:(i) a first desorption step includingpassing a heated desorbent liquid through a first contacting zonecontaining a solid adsorbent resin having a product adsorbed thereto, soas to desorb adsorbed product from the resin and form aproduct-containing medium; (ii) a heat exchange step after said firstdesorption step, including passing the product-containing medium througha heat exchange zone in which additional heat is transferred to themedium; and (iii) a second desorption step after said heat exchangestep, including passing the product-containing medium through the sameor another contacting zone containing solid adsorbent resin having anadditional amount of the product adsorbed thereto, to desorb the productfrom the resin and enrich the product-containing medium in the product;(b) including a portion of the product from step (a) in a liquid streampassed through a further contacting zone containing a solid adsorbentresin having an additional amount of the product adsorbed thereto. 17.The process of claim 16 wherein the product is a carboxylic acid andwherein in said second desorption step the product-containing medium ispassed through another contacting zone.
 18. The process of claim 17wherein the desorbent liquid is an aqueous medium.
 19. The process ofclaim 18 wherein the product is citric acid or lactic acid and thedesorbent liquid is water.
 20. The process of claim 19 wherein theproduct is citric acid.
 21. The process of claim 16 wherein during saidsecond desorption step, the heated desorbent liquid is passed throughthe same contacting zone as it was in said first desorption step,whereby heat transferred to the desorbent medium in said heat exchangestep is transferred to the solid adsorbent during said second desorptionstep.
 22. A thermal desorption process according to claim 16,comprising:(a) providing a plurality of chambers having inlet ports andoutlet ports and containing a solid adsorbent resin loaded with product;(b) advancing the chambers sequentially past a plurality of supply portsto cooperate with the inlet ports and discharge ports to cooperate withthe outlet ports; (c) introducing a heated desorbent liquid into a firstof the chambers through a first of the supply ports, the desorbentliquid passing over the adsorbent resin in the first chamber and exitingthrough a first of the discharge ports as a first product-containingmedium; (d) passing the first product-containing medium after step (c)through a heat exchange zone wherein it is heated; (e) conducting theheated medium after step (d) through a second of the supply and into asecond of the chambers, the product-containing medium passing over theloaded adsorbent resin in the second chamber and exiting through asecond of the discharge ports as a second product-containing mediumenriched in product as compared to the first product-containing medium.23. The process of claim 22 wherein said product is a carboxylic acid.24. The process of claim 23 wherein the product is citric acid.
 25. Theprocess of claim 22 wherein the desorbent liquid is an aqueous medium.26. The process of claim 22 wherein said supply ports cooperate withsaid inlet ports and said discharge ports cooperate with said outletports so as to pass the desorbent liquid through said chambers in acountercurrent fashion.
 27. The process of claim 22 wherein step (i)includes conducting the product-rich desorbent liquid after step (h)through a fourth and a fifth of said supply ports which precede saidthird supply port, and respectively into a fourth and fifth of saidchambers, said desorbent liquid passing over and heating the adsorbentresin in said fourth and fifth chambers.
 28. The process of claim 25wherein said product is a carboxylic acid.
 29. The process of claim 27wherein the carboxylic acid is citric acid or lactic acid and thedesorbent liquid is water.
 30. The process of claim 23 wherein thedesorbent liquid is an aqueous medium.
 31. The process of claim 30wherein said supply ports cooperate with said inlet ports and saiddischarge ports cooperate with said outlet ports so as to pass thedesorbent liquid through said chambers in a countercurrent fashion. 32.The process of claim 31 wherein said product is a carboxylic acid. 33.The process of claim 32 wherein the carboxylic acid is citric acid orlactic acid and the desorbent liquid is water.
 34. The process of claim21, wherein at least a portion of the desorbent liquid after step (e) ispassed again through the heat exchange zone of step (d) wherein it iscooled, and wherein the desorbent liquid is thereafter conducted againthrough said second chamber so as to cool the solid adsorbent resintherein.
 35. A desorption process, comprising:establishing a processwherein a plurality of contacting zones containing adsorbent aresequentially processed, the processing including substantially loadingthe adsorbent in the contacting zone with an adsorbed product, rinsingthe adsorbent in the contacting zone with a rinse agent, and thentreating the adsorbent in the contacting zone with a desorbing agent toform a product-containing medium; wherein a portion of theproduct-containing medium from a prior-processed contacting zone isincluded in the rinse agent in the processing of a subsequent contactingzone to decrease removal of the adsorbed product from the adsorbentduring the rinsing step.
 36. The process of claim 35 wherein the productis a carboxylic acid.
 37. The process of claim 36 wherein the desorbingagent is an aqueous medium.
 38. The process of claim 37 wherein theproduct is citric acid or lactic acid and the desorbing agent is water.39. The process of claim 38 wherein the product is citric acid.
 40. Theprocess of claim 36 wherein the adsorbent is a cross-linked polymerresin containing tertiary amine groups.
 41. The process of claim 38wherein the adsorbent is a crosslinked polymer resin containing tertiaryamine groups.
 42. The process of claim 41 wherein the adsorbent is apyridine-containing polymer resin.
 43. The process of claim 41 whereinthe polymer resin is crosslinked with divinylbenzene.
 44. The process ofclaim 42 wherein the polymer resin is crosslinked with divinylbenzene.45. The process of claim 44 wherein the polymer resin is adivinylbenzene crosslinked poly-2- or poly-4-vinylpyridine.
 46. Theprocess of claim 45 wherein the resin is in a macroreticular bead form.47. The process of claim 46 wherein the resin is about 2% to 25% byweight crosslinked with divinylbenzene.
 48. The process of claim 47wherein the product is citric acid.
 49. The process of claim 35,comprising:(a) providing a plurality of chambers having inlet ports andoutlet ports and containing a solid adsorbent resin loaded with adsorbedproduct; (b) advancing the chambers sequentially past a plurality ofsupply ports to cooperate with the inlet ports and discharge ports tocooperate with the outlet ports; (c) introducing a desorbent liquid intoa first of the chambers through a first of the supply ports, thedesorbent liquid passing over the adsorbent resin in the first chamberand exiting through a first of the discharge ports as aproduct-containing medium; (d) conducting the product-containing mediumafter step (c) through a second of the supply ports which precedes thefirst supply port, and into a second of the chambers, the desorbentliquid passing over and rinsing the adsorbent resin in the secondchamber and exiting through a second of the discharge ports.